Lighting devices are essential for our daily lives. In recent years, the global economic and commercial activities become more frequent. For improving the quality of home life, the power consumption associated with the lighting devices is gradually increased. For example, the widely-used lighting device is a low pressure gas discharge lamp such as a fluorescent lamp. For achieving the power-saving efficacy, more researchers devote themselves to reduce the power consumption of the low pressure gas discharge lamp. Moreover, the driving circuit of the lighting device is insufficient to meet the diverse requirements. Nowadays, the electronic ballast is designed to have many benefits such as low electromagnetic interference, high efficiency, high power correction factor, no flicker, low weight, high lighting quality and low power consumption.
Generally, the electronic ballasts are classified into two types, i.e. a current-preheat electronic ballast and a voltage-preheat electronic ballast. The conventional current-preheat electronic ballast can provide good starting time sequence to the fluorescent lamp and provide two frequency bands to the fluorescent lamp through a control chip (e.g. a ST L6574 chip). In a case that the current-preheat electronic ballast is operated at a higher frequency band, the lamp filaments of the fluorescent lamp is preheated, wherein the electric energy required to preheat the fluorescent lamp is provided by a resonant circuit of the electronic ballast. Whereas, in a case that the current-preheat electronic ballast is operated at a lower frequency band, the operating current of the fluorescent lamp is stably provided by the electronic ballast.
After the fluorescent lamp is normally operated, the current-preheat electronic ballast will continuously output a stable constant current in order to maintain the luminance of the fluorescent lamp. However, once the operating current flows through the lamp filament of the fluorescent lamp, a voltage drop across the two ends of the lamp filament is generated. Consequently, in a case that the current-preheat electronic ballast is applied to the widely-used fluorescent lamp with low lamp filament impedance (e.g. 2˜5 ohms), the voltage drop across the two ends of the lamp filament may be lower than a threshold voltage value (e.g. 4V). Under this circumstance, the life of the lamp filament is not obviously affected. Whereas, in a case that the current-preheat electronic ballast is applied to the high-efficiency fluorescent lamp with high lamp filament impedance (e.g. 8˜15 ohms), the voltage drop (e.g. 16V) across the two ends of the lamp filament will be higher than the threshold voltage value. Under this circumstance, the power consumption is increased, the use life of the fluorescent lamp is reduced, and the high-efficiency fluorescent lamp is possibly burnt out.
Therefore, there is a need of providing an improved current-preheat electronic ballast so as to obviate the above drawbacks.